Aptamers are referred to nucleic acids (DNA, RNA, and PNA) having binding affinity to specific substances. The substances to be specifically bound by the aptamers (hereinafter, referred to as target substances) are known to cover a wide spectrum of substances including biomolecules such as proteins, hormones, and peptides, artificial molecules such as agricultural chemicals, and small molecules such as potassium ions. Thus, level of the target substance contained in a sample can be quantified by detection of the binding of each aptamer to its target substance. Also, a sensor specifically responding to the target substance can be constructed by means of outputting the binding of the aptamer to the target substance as electric signals.
In the case when the detection of the target substance is carried out by using such an aptamer, such approach in which the formation and dissolution of a double-stranded nucleic acid portion between the aptamer and its complementary nucleic acid sequence is used as a measure for detection, because of advantages such as the applicability of this approach to aptamers having various steric structures (see e.g., Non Patent Document 1 and Patent Document 1).
For example, Non Patent Document 1 describes a method for detecting ATP in which an ATP aptamer 101 labeled with a fluorescent material 103 and a complementary strand 102 labeled with a quencher 104 for the fluorescence of the fluorescent material 103 are used for detection (FIGS. 1(a) to 1(c)). In the absence of ATP, the ATP aptamer 101 and its complementary strand 102 form a double-stranded nucleic acid portion 105 through their nucleotide sequences complementary to each other (FIG. 1(a)). When an ATP 107 is added to this double-stranded nucleic acid portion 105 (double-strand formation region), the complementary base pairs composing the double-strand formation region are dissociated in association with the binding of the ATP 107 to the ATP aptamer 101, resulting in the dissolution of the double-stranded nucleic acid portion 105 (double-strand formation region) (FIGS. 1(b) and 1(c)). When the double-stranded nucleic acid portion 105 is formed, the fluorescent material 103 is located in proximity to the quencher 104. When the double-stranded nucleic acid portion 105 is dissolved, the quencher 104 is placed distant from the fluorescent material 103. In such a case, the presence or absence of the double-stranded nucleic acid portion 105 can be detected by use of fluorescence resonance energy transfer (FRET). Thus, the presence or absence of the double-stranded nucleic acid portion 105 can be detected, as described therein, to thereby detect the presence or absence of the target substance ATP 107.
Alternatively, Patent Document 1 describes a sensor for detecting the presence or absence of a target substance by detecting the presence or absence of a double-stranded nucleic acid portion constituted by an aptamer immobilized on a substrate and its complementary strand. For example, as shown in FIG. 2, an aptamer 101 is immobilized on a substrate 108, and a complementary strand 102 having a labeling material 106 is hybridized thereto to form a double-stranded nucleic acid portion 105 (FIG. 2(a)). When a target substance 110 is added thereto, the double-stranded nucleic acid portion 105 is dissolved in association with the binding between the target substance 110 and the aptamer 101 to dissociate therefrom the complementary strand 102 having the labeling material 106 (FIG. 2(b)). The change in physical or chemical properties caused by the dissociation of the complementary strand 102 having the labeling material 106 is detected to thereby detect the formation of the binding between the target substance 110 and the aptamer 101. As a result of, for example, forming the binding of an aptamer 101 to a target substance 110, the separation of a complementary strand 102 having a metal particle 106 from a surface plasmon resonance sensor substrate 108 can be detected, as described therein, to thereby detect the presence of the target substance 110.